This invention relates to a thermal printer, and particularly to a thermal printer equipped with a mechanism for preventing a recording sheet's meandering.
In the case where data are recorded on a recording sheet with a thermal line head, the heat generating elements of the thermal line head locally heats the recording sheet to cause the latter to color. Therefore, when the printing density is high, the temperature of the recording sheet passing under the thermal printer head is raised to a considerably high value. As a result, the recording sheet is softened and thermally expanded to some extent.
This will be described in more detail. In the case where a pattern not in uniform in coloring density is printed with a thermal printer having a line head, the recording sheet is subjected to non-uniform thermal expansion and contraction, and therefore tension acting on the recording sheet becomes non-uniform in the widthwise direction of the recording sheet. Furthermore, the thermally expanded part or parts of the recording sheet are somewhat softened, thus being liable to be more expanded by the tension. As a result, the recording sheet passing through the printer head tends to move towards the side which is less expanded; that is, the recording sheet meanders. As a result, the printing position is displaced, and sometimes the recording sheet is creased, so that the resultant print is unsatisfactory.
For the purpose of preventing the recording sheet from meandering, heretofore the following methods are employed. For every printing operation, the line head is lifted to return the recording sheet to its correct position. Alternatively, a sheet edge position control device is employed which detects the position of the edge of the recording sheet so that, when the recording sheet is shifted, it is returned to its correct position. However, these conventional methods are disadvantageous in that the thermal printer is unavoidably high in manufacturing cost and large in size.
In addition, the following method has been known in the art: A roll with pins is used in such a manner that the pins are engaged with the perforations o a recording sheet to forcibly position the recording sheet thereby to prevent the meandering operation. However, the method is also disadvantageous in that, when the recording sheet is caused to meander greatly, the perforations are damaged.